The invention relates to a self-tapping screw with a boring part and a threaded shaft for fixing a panel-type object to a substructure. The screw is configured so that it produces a through hole in the panel-type object that accommodates the threaded shaft with a degree of play. The screw is also configured so that it produces a bore hole in the substructure, in which the thread of the shaft cuts a counter-thread.
For the production of building paneling, typically panels are attached by screws to a substructure rigidly connected to the building. The panels consist of, e.g., ceramic, asbestos cement, stone, or a similar brittle material. The building paneling typically consists of metal, frequently of profiled sections made from thin sheet metal. The building paneling is exposed to large temperature fluctuations. When the sun is shining, the building paneling expands greatly relative to the substructure, which is covered by the building paneling, and then contracts again as the sunshine diminishes. Thus, the building paneling wanders on the substructure. To reduce thermal stresses in the building paneling, the attachment screws must not hinder the movements of the paneling. For this purpose, through holes, which accommodate the attachment screws with a sufficient degree of play, are provided in the panels. In the substructure itself, threaded bore holes are produced, in which the attachment screws are screwed.
For smaller building panels, the through holes can be produced in the panels and the threaded holes can be produced in the substructure by drilling or pre-drilling and then manually cutting the threads at the construction site. For larger building panels, self-tapping screws are used, which have a boring part and a shaft provided with a cutting thread. The self-tapping screws are configured so that they themselves produce in the panel-type object a through hole, which accommodates the threaded shaft with a degree of play, and so that they themselves produce in the substructure a bore hole, in which the thread of the shaft cuts a counter-thread. A self-tapping screw of the above-mentioned type, like that known from EP 0 949 218 B1, is suitable for this purpose. For this screw, freely projecting wings connect to a panel-type boring part at an axial distance from the boring tip perpendicular to the screw axis. The edge of the blades facing the boring tip is formed as a boring knife. At the connecting point of the wings to the boring part, there is a predetermined breaking notch extending approximately parallel to the screw axis. The boring tip of the boring part first drills a hole in a cover track made from light metal to be attached to a substructure. The hole is then expanded by the boring wings into a through hole accommodating the screw with a degree of play. Then the boring part with the boring tip penetrates into the substructure, wherein, after the passage of the boring part, the wings contact the substructure and break off at the predetermined breaking notches. For further rotation of the screw, the thread part of the shaft is then guided into the bore hole produced in the substructure, so that a thread is cut into the substructure and the final attachment can be realized. For the use of such self-tapping screws for the production of building paneling on a substructure made from thin sheet metal profiled sections, it has been shown that frequently the torque exerted on the wings by the thin sheet metal substructure is not enough to break off the wings. Consequently, the wings also penetrate and expand the bore hole in the substructure, so that the thread of the shaft cannot be held in the substructure.